DE 196 19 169 C2 describes an electrochemical gas sensor with at least two electrodes, with an electrode support and with an electrolyte in a housing made of a material impermeable to the electrolyte: the otherwise closed housing has only one inlet and outlet capillary each for the gas to be measured. Furthermore, a heater and the working electrode are arranged in the housing in the flow path between the inlet capillary and the outlet capillary, wherein the heater forms a gap with the working electrode, and wherein the electrode array with the counterelectrode or additionally with the reference electrode is arranged on the side facing away from the gap in relation to the working electrode.
EP 1 326 075 A1 pertains to a device for measuring the alcohol vapor concentration in a sample, wherein the device has an electrochemical sensor for monitoring the alcohol by a diffusion current, means for determining the limiting diffusion current and means for generating an alcohol vapor concentration signal from the limiting diffusion current determined.
WO 92/22813 A1 describes a breath alcohol analyzing device with a sample chamber for collecting a breath sample, with a breath alcohol detector and with electronic means for processing data from the sample chamber and from the breath alcohol detector. The device has, besides, means for heating the sample chamber and/or the breath alcohol detector. The device can be used to influence the operation of a vehicle if the alcohol in the breath sample exceeds a preset limit.
In prior-art devices for measuring the breath alcohol concentration (alcohol-measuring device or breath alcohol-measuring device for short), the alcohol concentration in the air expired by a test subject is measured by means of indirect measurement methods, for example, by the electrochemical reaction of the alcohol molecules contained in the expired air in a measuring cell (electrochemical sensor). The method for breath alcohol determination by means of an electrochemical sensor is based on the principle of a fuel cell. For example, a piston (pump) driven by an electric motor delivers for this an air sample of the expired air with an exactly set volume (e.g., 1 cm3) into a sampling chamber. The electrochemical sensor analyzes the breath sample with the highest possible accuracy for the quantity of ethanol contained in the breath sample. A measuring electrode, a counterelectrode, a membrane located between these electrodes and a small quantity of electrolyte are present in the sensor (i.e., the sensor housing). The electrolyte and the electrode material are selected to be such that the substance to be analyzed (alcohol or ethanol) is electrochemically oxidized on the catalyst layer of the measuring electrode. The electrons being released during the reaction now flow over the connection wires of the sensor (i.e., connection wires of the measuring electrode and of the counterelectrode) as an electrical current to an analyzing unit of the alcohol-measuring device, where the electrode current can be measured, for example, over a load resistance. The total electric charge reacted in the electrochemical reaction is determined during the analysis of the sensor current. The sensor current is integrated over time in the case of this so-called coulometry, and the time integral corresponds to the area under the sensor current curve.
For example, a pressure sensor is used to determine the breath volume of the test subject. A dynamic pressure, which increases with increasing respiratory flow, develops due to the respiratory flow in front of a narrowing (diaphragm) in the gas path in the measuring device (e.g., in the mouthpiece). This dynamic pressure is determined with the pressure sensor and is an indicator of the value of the respiratory flow.
At low temperatures (e.g., <0° C.), the above-described electrochemical sensor of an alcohol-measuring device must at first be heated in order to make possible a reliable measurement of the alcohol concentration in the air expired by the test subject, because the rate of the chemical reaction in the sensor decreases greatly with decreasing temperature and thus makes it difficult to accurately analyze the sensor signals. The temperature of the sensor should be at least 0° C. for a reliable measurement. The necessary heating of the electrochemical sensor was carried out hitherto by placing a single heating element on the rear side of the sensor.
However, it was found that even if the sensor has reached its necessary temperature of at least 0°, the sensor yields sufficiently accurate measurement results only a certain time period after that temperature had been reached in prior-art alcohol-measuring devices. Consequently, it is necessary to wait a relatively long time after switching on the measuring device (i.e., after switching on the heating element to heat the sensor) before the measuring device becomes ready to operate in order to make it possible to carry out a reliable measurement of the breath alcohol concentration. This is felt to be very disturbing especially in case of interlock systems in motor vehicles, in which the engine of the vehicle can be started only after a breath test for measuring the breath alcohol concentration. There was no explanation so far for this undesired, long delay until the operational readiness is reached.